1. Field of the Invention
The present invention relates generally to nuclear reactor fuel assemblies and more particularly to an improved spacer grid assembly for spacing and cradling fuel rods in a nuclear fuel assembly.
2. Background
In a nuclear fuel assembly, spacer grid assemblies are used to precisely maintain the spacing between the fuel rods in a nuclear reactor core, to prevent rod vibration, and to provide lateral support for the fuel rods. Conventional spacer grid assembly designs include a multiplicity of metal grid straps, interlocked into an egg-crate configuration designed to form cells through which fuel rods (standard cells) and control rod guide thimbles (thimble cells) pass. Slots are often utilized to effect the interlocking engagement between grid straps. Each standard cell provides support for one fuel rod at a given axial location through the use of relatively resilient grid springs and relatively rigid protuberancies (dimples) formed in or attached to the metal grid straps. A peripheral strap may be used to enclose the interlocked grid straps in order to impart strength and rigidity to the fuel assembly and to provide grid spring and dimple locations for the peripheral cells. In order to minimize the lateral displacement of fuel rods during operation and to improve the fuel characteristics of a fuel assembly, a number of such spacer grid assemblies may be spaced along the fuel assembly length. These grids are generally held in place by attachment to the control rod guide thimbles by well known bulging techniques or the like.
Examples of such spacer grid assemblies may be found in U.S. Pat. Nos. 3,389,056 to E. Frisch, 3,713,971 to Van Santen et al., 3,944,467 to Biermann et a1., or 4,224,107 to Delafosse et al.
During operation in a nuclear reactor, the grid springs and dimples undergo intense radiation which cause the grid springs to tend to lose the initial spring force exerted against the fuel rods, thus permitting the fuel rods to vibrate and chatter against the dimples. The result may be fretting of the fuel rod cladding. In a typical fuel rod, the cladding comprises a metal tube that confines nuclear fuel pellets in the fuel rod. As used herein, the term "fretting" means a rubbing action between the fuel rod cladding and the grid springs and dimples. If severe enough, fretting is believed to result in fuel rod failure. In a nuclear fuel assembly, fretting may be caused by vibration of the rods induced by coolant and/or moderator fluid flow in the reactor core and possibly by vibration forces during shipping and insertion forces during fuel rod loading. Further, as will be understood by the artisan, fuel rods are generally long (several yards), slender cylinders. The round surface of these slender cylinders, when inserted into conventional spacer grid assemblies during fuel loading, typically rests on a flat or raised surface of conventional dimples and grid springs resulting in a point or line contact between the fuel rod surface and those springs and dimples. When loading fuel rods into such spacer grid assemblies, scratches known as loading scratches are a common problem. It is believed that when the fuel rods are inserted into such conventional spacer grid assemblies, the sharp edge on the dimples and springs tends to carve longitudinal scratches into the cladding.
Some conventional spacer grids contribute to an additional problem in that the fuel rod springs and dimples may not accurately position the fuel rod at the center of a standard cell. Deviations from that center position can result in adverse nuclear characteristics in the fuel assembly as well as hydraulic flow maldistribution and mechanical damage such as bent dimples and grid springs.